Axial flow water turbine systems that extract energy from flowing water are referred to herein as “current turbines.” Current turbines usually contain a propeller-like device or “rotor,” that is directed to receive a moving stream of water. As depicted in FIG. 1, a rotor can be either unshrouded or contained in a shroud. As the current hits the rotor, the current produces a force on the rotor in such a manner as to cause the rotor to rotate about its center. The rotor can be connected to either an electric generator or mechanical device through linkages such as gears, belts, chains or other means. Such turbines can be used for generating electricity and/or to drive rotating pumps or moving machine parts. They may also be used in large electricity generating “current turbine farms” (also termed “current turbine arrays”) containing multiple such turbines in a geometric pattern designed to allow maximum power extraction with minimal impact of each such turbine on one another and/or the surrounding environment.
The ability of an unshrouded rotor to convert fluid power to rotating power, when placed in a stream of width and depth larger than its diameter, is limited by the well documented theoretical value of 59.3% of the oncoming stream's power, known as the “Betz” limit which was documented by A. Betz in 1926. This productivity limit applies especially to the traditional multi-bladed axial current and tidal turbines shown in FIG. 1A. Attempts have been made to try to increase current turbine performance potential beyond the “Betz” limit. Properly designed shrouds can cause the oncoming flow to speed up as it approaches the rotor compared to what is experienced by an unshrouded rotor. The oncoming flow is thereby concentrated into the center of the duct. In general, for a properly designed rotor, this increased flow speed over that of an unshrouded rotor causes more force on the rotor and subsequently higher levels of power extraction than the same size unshrouded rotor. Previous shrouded current turbines such as those shown in FIG. 1B have employed entrance concentrators and exit diffusers to increase the flow velocities at the turbine rotor. Diffusers, which typically include a pipe-like structure with openings along the axial length to allow slow, diffusive mixing of water inside the pipe with that outside the pipe, generally require long lengths for good performance, and tend to be very sensitive to oncoming flow variations. Such long, flow sensitive diffusers are impractical in many installations. Short diffusers can stall and thereby reduce the energy conversion efficiency of the system.